FMAC FirstMark Horizon Acquisition Corp

Filed by Starry Group Holdings, Inc.

Pursuant to Rule 425 under the Securities Act of 1933

and deemed filed pursuant to Rule 14a-12

under the Securities Exchange Act of 1934

Subject Company: FirstMark Horizon Acquisition Corp.

Commission File No.: 001-39585

Date: March 11, 2022

FirstMark Horizon Acquisition Corp. Reminds Stockholders of Pro Rata Right to Additional Shares for Non-Redemption and That Voting Has Commenced for Business Combination with Starry, Inc.

Creates additional value for non-redeeming stockholders, and stockholders are encouraged to vote “FOR” the business combination and related proposals

March 11, 2022 08:55 AM Eastern Standard Time

NEW YORK—(BUSINESS WIRE)—FirstMark Horizon Acquisition Corp. (“FMAC” or “FirstMark”) (NYSE: FMAC) reminds its stockholders of the structure in its proposed business combination with Starry, Inc. (“Starry”) and Starry Group Holdings, Inc. (“Starry Group Holdings”) that provides non-redeeming stockholders with a pro rata right to a portion of additional shares of the post-business combination company and reminds FirstMark stockholders to vote “FOR” the proposed business combination with Starry and Starry Group Holdings.

Pro Rata Share and Value Distribution to Non-Redeeming Stockholders

The proposed business combination with Starry is structured to provide FMAC stockholders that do not redeem their shares of FMAC common stock with a right to a pro rata portion of an additional 1,000,000 shares of Class A common stock, par value $0.0001 per share, of Starry Group Holdings, the post-business combination public company (“Starry Group Holdings Class A Common Stock”). These shares of Starry Group Holdings Class A Common Stock have been registered pursuant to the registration statement on Form S-4 filed with the Securities and Exchange Commission on November 5, 2021, as amended and supplemented from time to time, and will be delivered by the exchange agent to non-redeeming stockholders of FMAC upon the consummation of the proposed business combination.

These pro rata shares result in additional value being delivered to non-redeeming stockholders based on the final Class A Exchange Ratio of between 1.0242 and 1.2415, which depends on the overall level of redemptions. The table below represents possible scenarios based on a $10 per share price:

To invest in Starry, individuals can buy public shares of FMAC and hold them through the closing of the proposed business combination, at which time their FMAC public shares will automatically convert to Starry Group Holdings Class A Common Stock on a greater than 1:1 basis.

Voting for the Business Combination

Holders of FMAC common stock as of the record date (February 3, 2022) may attend the Special Meeting and vote their shares electronically during the Special Meeting. The Special Meeting will be held via live webcast at https://www.cstproxy.com/firstmarkhorizon/2022. Such holders will need the meeting control number that is printed on their proxy card to enter the Special Meeting. Such holders may also attend the meeting telephonically by dialing (800) 450-7155.

About Starry, Inc.

At Starry we believe the future is built on connectivity and that connecting people and communities to high-speed, broadband internet should be simple and affordable. Using our innovative, wideband hybrid-fiber fixed wireless technology, Starry is deploying gigabit capable broadband to the home without bundles, data caps, or long-term contracts. Starry is a different kind of internet service provider. We’re building a platform for the future by putting our customers first, protecting their privacy, ensuring access to an open and neutral net, and making affordable connectivity and digital equity a priority. Headquartered in Boston and backed by world-class investors, Starry is currently available in Boston, New York City, Los Angeles, Washington, DC, Denver and Columbus and is expanding nationwide. To learn more about Starry or to join our team and help us build a better internet, visit: https://starry.com.

About FirstMark Horizon Acquisition Corp.

FirstMark is a special purpose acquisition company whose mission is to drive long-term value creation by actively supporting the next generation of iconic public companies. FirstMark is comprised of a team of seasoned investors and industry executives with an extensive track record of identifying transformative trends across innovative subsectors of technology. Notably, FirstMark’s management team is composed of the founders and executives of FirstMark Capital, a prominent technology venture capital firm founded in 2008 with $2.3 billion in total capital commitments, which has backed entrepreneurs that have created leading companies, many valued at over a billion dollars. For more information, please visit http://firstmarkhorizon.com.

Additional Information about the Business Combination and Where to Find It

In connection with the proposed business combination, Starry Group Holdings has filed a registration statement on Form S-4 (the “Form S-4”) with the Securities and Exchange Commission (the “SEC”) on November 5, 2021 (as amended on December 20, 2021, January 14, 2022, February 4, 2022 and February 9, 2022). The Form S-4 includes a proxy statement of FirstMark and a prospectus of Starry Group Holdings, referred to as a proxy statement/prospectus. The Form S-4 has been declared effective by the SEC and the definitive proxy statement/prospectus, including a supplement to the definitive proxy statement/prospectus filed on March 7, 2022 and mailed to all FirstMark stockholders. Additionally, Starry Group Holdings and FirstMark have also filed and may continue to file other relevant materials with the SEC in connection with the proposed business combination, including a supplement to the definitive proxy statement/prospectus filed on March 7, 2022 and mailed to all FirstMark stockholders. Copies of the Form S-4, the definitive proxy statement/prospectus and all other relevant materials filed or that will be filed with the SEC by FirstMark or Starry Group Holdings may be obtained free of charge at the SEC’s website at www.sec.gov. Before making any voting or investment decision, investors and security holders of FirstMark are urged to read the Form S-4, the definitive proxy statement/prospectus and all other relevant materials filed or that will be filed with the SEC in connection with the proposed business combination because they will contain important information about the proposed business combination and the parties to the proposed business combination.

2

Participants in Solicitation

FirstMark, Starry Group Holdings and Starry and their respective directors and executive officers, under SEC rules, may be deemed to be participants in the solicitation of proxies of FirstMark’s stockholders in connection with the proposed business combination. Investors and security holders may obtain more detailed information regarding the names and interests in the proposed business combination of FirstMark’s directors and officers in FirstMark’s filings with the SEC, including FirstMark’s registration statement on Form S-1, which was originally filed with the SEC on September 18, 2020. To the extent that holdings of FirstMark’s securities have changed from the amounts reported in FirstMark’s registration statement on Form S-1, such changes have been or will be reflected on Statements of Change in Ownership on Form 4 filed with the SEC. Information regarding the persons who may, under SEC rules, be deemed participants in the solicitation of proxies to FirstMark’s stockholders in connection with the business combination is included in the definitive proxy statement/prospectus relating to the proposed business combination. You may obtain free copies of these documents as described in the preceding paragraph.

No Offer or Solicitation

This communication shall not constitute a proxy statement or solicitation of a proxy, consent or authorization with respect to any securities or in respect of the proposed business combination. This communication shall also not constitute an offer to sell or a solicitation of an offer to buy any securities of FirstMark, Starry Group Holdings or Starry, nor shall there be any sale of securities in any state or jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such state or jurisdiction. No offering of securities shall be made except by means of a prospectus meeting the requirements of Section 10 of the Securities Act of 1933, as amended.

Forward-Looking Statements

Certain statements made in this communication are “forward-looking statements” within the meaning of the “safe harbor” provisions of the Private Securities Litigation Reform Act of 1995 with respect to the proposed business combination between FirstMark and Starry. Words such as “expect,” “estimate,” “project,” “budget,” “forecast,” “anticipate,” “intend,” “plan,” “may,” “will,” “could,” “should,” “believe,” “predict,” “potential,” “continue,” “strategy,” “future,” “opportunity,” “would,” “seem,” “seek,” “outlook” and similar expressions are intended to identify such forward-looking statements. Forward-looking statements are predictions, projections and other statements about future events that are based on current expectations and assumptions and, as a result, are subject to risks and uncertainties that could cause the actual results to differ materially from the expected results. These statements are based on various assumptions, whether or not identified in this communication. These forward-looking statements are provided for illustrative purposes only and are not intended to serve as, and must not be relied on by an investor as, a guarantee, an assurance, a prediction or a definitive statement of fact or probability. Actual events and circumstances are difficult or impossible to predict and will differ from assumptions. These forward-looking statements include, without limitation, Starry’s and FirstMark’s expectations with respect to the anticipated financial impacts of the proposed business combination, the satisfaction of closing conditions to the proposed business combination, and the timing of the completion of the proposed business combination. You should carefully consider the risks and uncertainties described in the “Risk Factors” section of FirstMark’s registration statement on Form S-1 (File No. 333-248916), its Annual Report on Form 10-K, as amended from time to time, for the fiscal year ended December 31, 2020, and its subsequent Quarterly Reports on Form 10-Q. In addition, there are risks and uncertainties described in the definitive proxy statement/prospectus filed by Starry Group Holdings and other documents filed by FirstMark or Starry

3

Group Holdings from time to time with the SEC. These filings identify and address other important risks and uncertainties that could cause actual events and results to differ materially from those contained in the forward-looking statements. Most of these factors are outside Starry’s, Starry Group Holdings’ and FirstMark’s control and are difficult to predict. Many factors could cause actual future events to differ from the forward-looking statements in this communications, including but not limited to: (1) the outcome of any legal proceedings that may be instituted against FirstMark, Starry or Starry Group Holdings following the announcement of the proposed business combination; (2) the inability to complete the proposed business combination, including due to the inability to concurrently close the business combination and related transactions, including the private placements of common stock and convertible notes or due to failure to obtain approval of the stockholders of FirstMark; (3) the risk that the proposed business combination may not be completed by FirstMark’s business combination deadline and the potential failure to obtain an extension of the business combination deadline if sought by FirstMark; (4) the failure to satisfy the conditions to the consummation of the proposed business combination, including the approval by the stockholders of FirstMark, the satisfaction of the minimum trust account amount following any redemptions by FirstMark’s public stockholders and the receipt of certain governmental and regulatory approvals; (5) delays in obtaining, adverse conditions contained in, or the inability to obtain necessary regulatory approvals or complete regulatory reviews required to complete the proposed business combination; (6) the occurrence of any event, change or other circumstance that could give rise to the termination of the merger agreement; (7) volatility in the price of FirstMark’s, Starry’s or Starry Group Holdings’ securities; (8) the risk that the proposed business combination disrupts current plans and operations as a result of the announcement and consummation of the business combination; (9) the inability to recognize the anticipated benefits of the proposed business combination, which may be affected by, among other things, competition, the ability of the combined company to grow and manage growth profitably, maintain relationships with customers and suppliers and retain key employees; (10) costs related to the proposed business combination; (11) changes in the applicable laws or regulations; (12) the possibility that the combined company may be adversely affected by other economic, business, and/or competitive factors; (13) the risk of downturns and a changing regulatory landscape in the highly competitive industry in which Starry operates; (14) the impact of the global COVID-19 pandemic; (15) Starry’s ability to obtain or maintain rights to use licensed spectrum in any market in which Starry operates and potential declines in the value of Starry’s FCC licenses; (16) the potential inability of Starry to raise additional capital needed to pursue its business objectives or to achieve efficiencies regarding other costs; (17) the enforceability of Starry’s intellectual property, including its patents, and the potential infringement on the intellectual property rights of others, cyber security risks or potential breaches of data security; and (18) other risks and uncertainties described in FirstMark’s registration statement on Form S-1 and Annual Report on Form 10-K, as amended from time to time, for the fiscal year ended December 31, 2020 and its subsequent Quarterly Reports on Form 10-Q, and in the definitive proxy statement/prospectus filed by Starry Group Holdings. These risks and uncertainties may be amplified by the COVID-19 pandemic, which has caused significant economic uncertainty. Starry, Starry Group Holdings and FirstMark caution that the foregoing list of factors is not exclusive or exhaustive and not to place undue reliance upon any forward-looking statements, including projections, which speak only as of the date made. None of Starry, Starry Group Holdings or FirstMark gives any assurance that Starry, Starry Group Holdings or FirstMark will achieve its expectations. None of Starry, Starry Group Holdings or FirstMark undertakes or accepts any obligation to publicly provide revisions or updates to any forward-looking statements, whether as a result of new information, future developments or otherwise, or should circumstances change, except as otherwise required by securities and other applicable laws.

Contacts

Investors for Starry and Starry Group Holdings:

investors@starry.com

Investors for FirstMark:

Eric D. Cheung

eric@firstmarkcap.com

Media Contact:

Mimi Ryals, Starry

mryals@starry.com

press@starry.com

4